Operation Sea Lion

In September 1939, the successful[6] German invasion of Poland infringed on both a French and a British alliance with Poland and both countries declared war on Germany. On 9 October, Hitler's "Directive No. 6 for the Conduct of the War" planned an offensive to defeat these allies and "win as much territory as possible in Holland, Belgium, and northern France to serve as a base for the successful prosecution of the air and sea war against England".[7]

With the prospect of the Channel ports falling under Kriegsmarine (German Navy) control, Grand Admiral (Großadmiral) Erich Raeder (head of the Kriegsmarine) attempted to anticipate the obvious next step that might entail and instructed his operations officer, Kapitän Hansjürgen Reinicke, to draw up a document examining "the possibility of troop landings in England should the future progress of the war make the problem arise". Reinicke spent five days on this study and set forth the following prerequisites:

• Eliminating or sealing off Royal Navy forces from the landing and approach areas.
• Eliminating the Royal Air Force.
• Destroying all Royal Navy units in the coastal zone.
• Preventing British submarine action against the landing fleet.[8]

On 22 November 1939, the Head of Luftwaffe (German Air Force) intelligence Joseph "Beppo" Schmid presented his "Proposal for the Conduct of Air Warfare", which argued for a counter to the British blockade and said "Key is to paralyse the British trade" by blocking imports to Britain and attacking seaports. The OKW (Oberkommando der Wehrmacht or "High Command of the Armed Forces") considered the options and Hitler's 29 November "Directive No. 9 – Instructions For Warfare Against The Economy of the Enemy" stated that once the coast had been secured, the Luftwaffe and Kriegsmarine were to blockade UK ports with sea mines, attack shipping and warships, and make air attacks on shore installations and industrial production. This directive remained in force in the first phase of the Battle of Britain.[9]

British soldiers in France in 1939

In December 1939, the German Army issued its own study paper (designated Nordwest) and solicited opinions and input from both Kriegsmarine and Luftwaffe. The paper outlined an assault on England's eastern coast between The Wash and the River Thames by troops crossing the North Sea from ports in the Low Countries. It suggested airborne troops as well as seaborne landings of 100,000 infantry in East Anglia, transported by the Kriegsmarine, which was also to prevent Royal Navy ships from getting through the Channel, while the Luftwaffe had to control airspace over the landings. The Kriegsmarine response was focused on pointing out the many difficulties to be surmounted if invading England was to be a viable option. It could not envisage taking on the Royal Navy Home Fleet and said it would take a year to organise shipping for the troops. Reichsmarschall Hermann Göring, head of the Luftwaffe, responded with a single-page letter in which he stated, "[A] combined operation having the objective of landing in England must be rejected. It could only be the final act of an already victorious war against Britain as otherwise the preconditions for success of a combined operation would not be met".[10][11]

Germany's swift and successful occupation of France and the Low Countries gained control of the Channel coast, facing what Schmid's 1939 report called their "most dangerous enemy". Raeder met Hitler on 21 May 1940 and raised the topic of invasion, but warned of the risks and expressed a preference for blockade by air, submarines and raiders.[12][13]

By the end of May, the Kriegsmarine had become even more opposed to invading Britain following its costly victory in Norway; after Operation Weserübung, the Kriegsmarine had only one heavy cruiser, two light cruisers, and four destroyers available for operations.[14] Raeder was strongly opposed to Sea Lion, for over half of the Kriegsmarine surface fleet had been either sunk or badly damaged in Weserübung, and his service was hopelessly outnumbered by the ships of the Royal Navy.[15] British parliamentarians still arguing for peace negotiations were defeated in the May 1940 War Cabinet Crisis, but throughout July the Germans continued with attempts to find a diplomatic solution.[16]

In a report presented on 30 June, OKW Chief of Staff Alfred Jodl reviewed options to increase pressure on Britain to agree to a negotiated peace. The first priority was to eliminate the Royal Air Force and gain air supremacy. Intensified air attacks against shipping and the economy could affect food supplies and civilian morale in the long term. Reprisal attacks of terror bombing had the potential to cause quicker capitulation but the effect on morale was uncertain. Once the Luftwaffe had control of the air and the British economy had been weakened, an invasion would be a last resort or a final strike ("Todesstoss") after the UK had already been practically defeated, but could have a quick result.[12][17] At a meeting that day, OKH Chief of General Staff Franz Halder heard from Secretary of State Ernst von Weizsäcker that Hitler had turned his attention to Russia. Halder met Admiral Otto Schniewind on 1 July, and they shared views without understanding each other's position. Both thought that air superiority was needed first, and could make the invasion unnecessary. They agreed that minefields and U-boats could limit the threat posed by the Royal Navy; Schniewind emphasised the significance of weather conditions.[18]

On 2 July, the OKW asked the services to start preliminary planning for an invasion, as Hitler had concluded that invasion would be achievable in certain conditions, the first of which was command of the air, and specifically asked the Luftwaffe when this would be achieved. On 4 July, after asking General Erich Marcks to begin planning an attack on Russia, Halder heard from the Luftwaffe that they planned to eliminate the RAF, destroying its aircraft manufacturing and supply systems, with damage to naval forces as a secondary aim. A Luftwaffe report presented to the OKW at a meeting on 11 July said that it would take 14 to 28 days to achieve air superiority. The meeting also heard that England was discussing an agreement with Russia. On the same day, Grand Admiral Raeder visited Hitler at the Berghof to persuade him that the best way to pressure the British into a peace agreement would be a siege combining air and submarine attacks. Hitler agreed with him that invasion would be a last resort.[19]

Jodl set out the OKW proposals for the proposed invasion in a memorandum issued on 12 July, which described operation Löwe (Lion) as "a river crossing on a broad front", irritating the Kriegsmarine. On 13 July, Hitler met Field Marshal von Brauchitsch and Halder at Berchtesgaden and they presented detailed plans prepared by the army on the assumption that the navy would provide safe transport.[20] To the surprise of Von Brauchitsch and Halder, and completely at odds with his normal practice, Hitler did not ask any questions about specific operations, had no interest in details, and made no recommendations to improve the plans; instead he simply told OKW to start preparations.[21]

On 16 July 1940 Hitler issued Führer Directive No. 16, setting in motion preparations for a landing in Britain. He prefaced the order by stating: "As England, in spite of her hopeless military situation, still shows no signs of willingness to come to terms, I have decided to prepare, and if necessary to carry out, a landing operation against her. The aim of this operation is to eliminate the English Motherland as a base from which the war against Germany can be continued, and, if necessary, to occupy the country completely." The code name for the invasion was Seelöwe, "Sea Lion".[22][23]

Hitler's directive set four conditions for the invasion to occur:[24]

• The RAF was to be "beaten down in its morale and in fact, that it can no longer display any appreciable aggressive force in opposition to the German crossing".
• The English Channel was to be swept of British mines at the crossing points, and the Strait of Dover must be blocked at both ends by German mines.
• The coastal zone between occupied France and England must be dominated by heavy artillery.
• The Royal Navy must be sufficiently engaged in the North Sea and the Mediterranean so that it could not intervene in the crossing. British home squadrons must be damaged or destroyed by air and torpedo attacks.

This ultimately placed responsibility for Sea Lion's success squarely on the shoulders of Raeder and Göring, neither of whom had the slightest enthusiasm for the venture and, in fact, did little to hide their opposition to it.[25] Nor did Directive 16 provide for a combined operational headquarters, similar to the Allies' creation of the Supreme Headquarters Allied Expeditionary Force (SHAEF) for the later Normandy landings, under which all three service branches (Army, Navy, and Air Force) could work together to plan, co-ordinate, and execute such a complex undertaking.[26]

The invasion was to be on a broad front, from around Ramsgate to beyond the Isle of Wight. Preparations, including overcoming the RAF, were to be in place by mid August.[22][19]

Grand Admiral Raeder sent a memorandum to OKW on 19 July, complaining about the onus placed on the navy in relation to the army and air force, and stating that the navy would be unable to achieve its objectives.[20]

The first joint services conference on the proposed invasion was held by Hitler in Berlin on 21 July, with Raeder, Field Marshal von Brauchitsch, and Luftwaffe Chief of Staff Hans Jeschonnek. Hitler told them that the British had no hope of survival, and ought to negotiate, but were hoping to get Russia to intervene and halt German oil supplies. Invasion was very risky, and he asked them if direct attacks by air and submarine could take effect by mid September. Jeschonnek proposed large bombing attacks so that responding RAF fighters could be shot down. The idea that invasion could be a surprise "river crossing" was dismissed by Raeder, and the navy could not complete its preparations by mid August. Hitler wanted the air attack to commence early in August and, if it succeeded, the invasion was to start around 25 August before weather deteriorated. Hitler's main interest was the question of countering potential Russian intervention. Halder outlined his first thoughts on defeating Russian forces. Detailed plans were to be made to attack the Soviet Union.[27]

Raeder met Hitler on 25 July to report on navy progress: they were not sure if preparations could be completed during August: he was to present plans at a conference on 31 July. On 28 July he told OKW that ten days would be needed to get the first wave of troops across the Channel, even on a much narrower front. Planning was to resume. In his diary, Halder noted that if what Raeder had said was true, "all previous statements by the navy were so much rubbish and we can throw away the whole plan of invasion". On the next day, Halder dismissed the navy's claims and required a new plan.[28][29]

The Luftwaffe announced on 29 July that they could begin a major air attack at the start of August, and their intelligence reports gave them confidence of a decisive result. Half of their bombers were to be kept in reserve to support the invasion. At a meeting with the army, the navy proposed delay until May 1941, when the new battleships Bismarck and Tirpitz would be ready. A navy memorandum issued on 30 July said invasion would be vulnerable to the Royal Navy, and autumn weather could prevent necessary maintenance of supplies. The OKW assessed alternatives, including attacking the British in the Mediterranean, and favoured extended operations against England while remaining on good terms with Russia.[28]

At the Berghof conference on 31 July, the Luftwaffe were not represented. Raeder said barge conversions would take until 15 September, leaving the only possible 1940 invasion dates as 22–26 September, when weather was likely to be unsuitable. Landings would have to be on a narrow front, and would be better in spring 1941. Hitler wanted the invasion in September as the British army was increasing in strength. After Raeder left, Hitler told von Brauchitsch and Halder that the air attack was to start around 5 August; eight to fourteen days after that, he would decide on the landing operation. London was showing new-found optimism, and he attributed this to their hopes of intervention by Russia, which Germany was to attack in the spring of 1941.[30]

On 1 August 1940, through Führer Directive No.17, Hitler instructed[31] intensified air and sea warfare to "establish the necessary conditions for the final conquest of England". From 5 August, subject to weather delays, the Luftwaffe was "to overpower the English Air Force with all the forces at its command, in the shortest possible time." Attacks were then to be made on ports and food stocks, while leaving alone ports to be used in the invasion, and "air attacks on enemy warships and merchant ships may be reduced except where some particularly favourable target happens to present itself." The Luftwaffe was to keep sufficient forces in reserve for the proposed invasion, and was not to target civilians without a direct order from Hitler in response to RAF terror bombing. No decision had been reached on the choice between immediate decisive action and a siege. The Germans hoped the air action would force the British to negotiate, and make invasion unnecessary.[32][33]

The success of the German invasion of Denmark and Norway, on 9 April 1940, had relied extensively on the use of paratroop and glider-borne formations (Fallschirmjäger) to capture key defensive points in advance of the main invasion forces. The same airborne tactics had also been used in support of the invasions of Belgium and the Netherlands on 10 May 1940. However, although spectacular success had been achieved in the airborne assault on Fort Eben-Emael in Belgium, German airborne forces had come close to disaster in their attempt to seize the Dutch government and capital of The Hague. Around 1,300 of the 22nd Air Landing Division had been captured (subsequently shipped to Britain as prisoners of war), around 250 Junkers Ju 52 transport aircraft had been lost, and several hundred elite paratroops and air-landing infantry had been killed or injured. Consequently, even in September 1940 the Luftwaffe had the capacity to provide only around 3,000 airborne troops to participate in the first wave of Operation Sea Lion.

Winston Churchill visiting bomb-damaged areas of the East End of London, 8 September 1940

The Battle of Britain began in early July 1940, with attacks on shipping and ports in the Kanalkampf which forced RAF Fighter Command into defensive action. In addition, wider raids gave aircrew experience of day and night navigation, and tested the defences.[43] On 13 August, the German Luftwaffe began a series of concentrated aerial attacks (designated Unternehmen Adlerangriff or Operation Eagle Attack) on targets throughout the United Kingdom in an attempt to destroy the RAF and establish air superiority over Great Britain. The change in emphasis of the bombing from RAF bases to bombing London, however, turned Adlerangriff into a short-range strategic bombing operation.

The effect of the switch in strategy is disputed. Some historians argue the change in strategy lost the Luftwaffe the opportunity of winning the air battle or air superiority.[44] Others argue the Luftwaffe achieved little in the air battle and the RAF was not on the verge of collapse, as often claimed.[45] Another perspective has also been put forward, which suggests the Germans could not have gained air superiority before the weather window closed.[46] Others have said that it was unlikely the Luftwaffe would ever have been able to destroy RAF Fighter Command. If British losses became severe, the RAF could simply have withdrawn northward and regrouped. It could then deploy if the Germans launched an invasion. Most historians agree Sea Lion would have failed regardless because of the weakness of the German Kriegsmarine compared to the Royal Navy.[47]

The record of the Luftwaffe against naval combat vessels up to that point in the war was poor. In the Norwegian campaign, despite eight weeks of continuous air supremacy, the Luftwaffe sank only two British warships. The German aircrews were not trained or equipped to attack fast-moving naval targets, particularly agile naval destroyers or Motor Torpedo Boats (MTB). The Luftwaffe also lacked armour-piercing bombs[48] and their only aerial torpedo capability, essential for defeating larger warships, consisted of a small number of slow and vulnerable Heinkel He 115 floatplanes. The Luftwaffe made 21 deliberate attacks on small torpedo boats during the Battle of Britain, sinking none. The British had between 700 and 800 small coastal craft (MTBs, Motor Gun Boats and smaller vessels), making them a critical threat if the Luftwaffe could not deal with the force. Only nine MTBs were lost to air attack out of 115 sunk by various means throughout the Second World War. Only nine destroyers were sunk by air attack in 1940, out of a force of over 100 operating in British waters at the time. Only five were sunk while evacuating Dunkirk, despite large periods of German air superiority, thousands of sorties flown, and hundreds of tons of bombs dropped. The Luftwaffe's record against merchant shipping was also unimpressive: it sank only one in every 100 British vessels passing through British waters in 1940, and most of this total was achieved using mines.[49]

Had an invasion taken place, the Bf 110 equipped Erprobungsgruppe 210 would have dropped Seilbomben just prior to the landings. This was a secret weapon which would have been used to blackout the electricity network in south-east England. The equipment for dropping the wires was fitted to the Bf 110 aeroplanes and tested. It involved dropping wires across high voltage wires, and was probably as dangerous to the aircraft crews as to the British.[50]

Upon hearing of Hitler's intentions, Italian dictator Benito Mussolini, through his Foreign Minister Count Galeazzo Ciano, quickly offered up to ten divisions and thirty squadrons of Italian aircraft for the proposed invasion.[51] Hitler initially declined any such aid but eventually allowed a small contingent of Italian fighters and bombers, the Italian Air Corps (Corpo Aereo Italiano or CAI), to assist in the Luftwaffe's aerial campaign over Britain in October and November 1940.[52]

The Kriegsmarine invested considerable energy in planning and assembling the forces for an elaborate deception plan called Operation Herbstreise or "Autumn Journey". The idea was first mooted by Generaladmiral Rolf Carls on 1 August proposing a feint expedition into the North Sea resembling a troop convoy heading for Scotland, with the aim of drawing the British Home Fleet away from the intended invasion routes. Initially, the convoy was to consist of about ten small cargo ships fitted with false funnels to make them appear larger, and two small hospital ships. As the plan gathered momentum, the large ocean liners Europa, Bremen, Gneisenau and Potsdam were added to the list. These were organised into four separate convoys, escorted by light cruisers, torpedo boats and minesweepers, some of which were obsolete vessels being used by naval training bases. The plan was that three days before the actual invasion, the troopships would load the men and equipment of four divisions in major Norwegian and German ports and put to sea, before unloading them again on the same day in quieter locations. Returning to sea, the convoys would head west towards Scotland before turning around at about 21:00 on the following day. In addition, the only heavy warships available to the Kriegsmarine, the heavy cruisers Admiral Scheer and Admiral Hipper, would attack the British armed merchant cruisers of the Northern Patrol and convoys inbound from Canada; however, the Scheer's repairs overran and if the invasion had taken place in September, would have left the Hipper to operate alone.[57]

Lacking surface naval forces capable of meeting the Home Fleet of the Royal Navy in open battle, the main seaborne defence for the first wave invasion fleets would be four massive minefields, which were intended to be laid from S minus nine onwards. The ANTON minefield (off Selsey Bill) and the BRUNO minefield (off Beachy Head), each totalling over 3,000 mines in four rows, would block off the invasion beaches against naval forces from Portsmouth, while the counterpart CAESAR minefield would block off beach 'B' from Dover. A fourth minefield, DORA, was to be laid off Lyme Bay to inhibit naval forces from Plymouth. By the autumn of 1940, the Kriegsmarine had achieved considerable success in laying minefields in support of active operations, notably in the night of 31 August 1940 when the 20th Destroyer flotilla suffered heavy losses when running into a newly laid German minefield near the Dutch coast off Texel; however no plans were made to prevent the mines being cleared by the large force of British minesweepers which were based in the area. Vizeadmiral Friedrich Ruge, who was in charge of the mining operation, wrote after the war that if the minefields had been relatively complete, they would have been a "strong obstacle" but that "even a strong obstacle is not an absolute barrier."[58]

Invasion barges assembled at the German port of Wilhelmshaven.

In 1940 the German Navy was ill-prepared for mounting an amphibious assault the size of Operation Sea Lion. Lacking purpose-built landing craft and both doctrinal and practical experience with amphibious warfare, the Kriegsmarine was largely starting from scratch. Some efforts had been made during the inter-war years to investigate landing military forces by sea, but inadequate funding severely limited any useful progress.[59]

For the successful German invasion of Norway, German naval forces (assisted in places by thick fog) had simply forced an entry into key Norwegian harbours with motor launches and E-boats against stiff resistance from the outgunned Norwegian army and navy, and then unloaded troops from destroyers and troop transports directly onto the dockfronts at Bergen, Egersund, Trondheim, Kristiansand, Arendal and Horten.[60] At Stavanger and Oslo capture of the port was preceded by landing airborne forces. No beach landings were attempted.

A Pionierlandungsboot.

The Kriegsmarine had taken some small steps in remedying the landing craft situation with construction of the Pionierlandungsboot 39 (Engineer Landing Boat 39), a self-propelled shallow-draft vessel which could carry 45 infantrymen, two light vehicles or 20 tons of cargo and land on an open beach, unloading via a pair of clamshell doors at the bow. But by late September 1940 only two prototypes had been delivered.[61]

Recognising the need for an even larger craft capable of landing both tanks and infantry onto a hostile shore, the Kriegsmarine began development of the 220-ton Marinefährprahm (MFP) but these too were unavailable in time for a landing on British soil in 1940, the first of them not being commissioned until April 1941.

Given barely two months to assemble a large seagoing invasion fleet, the Kriegsmarine opted to convert inland river barges into makeshift landing craft. Approximately 2,400 barges were collected from throughout Europe (860 from Germany, 1,200 from the Netherlands and Belgium and 350 from France). Of these, only about 800 were powered albeit insufficiently to cross the Channel under their own power. All barges would be towed across by tugs, with two barges to a tug in line abreast, preferably one being powered and one unpowered. On reaching the English coast, the powered barges would be cast-off, to beach themselves under their own power; the unpowered barges would be taken inshore as far as possible by the tugs and anchored, so as to settle on the falling tide, their troops unloading some hours later than those on the powered barges.[62] Accordingly, the Sea Lion plans were prepared on the basis that the landings would take place shortly after high tide and on a date when this coincided with sunrise. Towards evening, on the following rising tide, the empty barges would have been retrieved by their tugs to receive the second echelon forces, stores and heavy equipment in the awaiting transport vessels. These transport vessels would have remained moored off the beach throughout the day. By contrast, the Allied D day landings in 1944 were timed to happen at low tide; with all troops and equipment transhipped from their transport vessels to landing craft off-shore overnight.

All the troops intended to land at beach 'E', the westernmost of the four beaches, would cross the channel in larger transport vessels – the barges being towed loaded with equipment but empty of troops – and would then be transferred onto their barges a short distance from the beach. For the landings on the other three beaches, the first echelon of the invasion forces (and their equipment) would be loaded onto their barges in French or Belgian ports, while the second echelon force crossed the channel in associated transport vessels. Once the first echelon had been unloaded onto the beach, the barges would return to the transport vessels to transport the second echelon. The same procedure was envisaged for the second wave (unless the first wave had captured a usable port). Trials showed that this process of trans-shipment in open sea, in any circumstances other than flat calm, would likely take at least 14 hours,[63] such that the disembarkation of the first wave might extend over several tides and several days, with barges and invasion fleet subsequently needing to be escorted together back across the Channel for repairs and reloading. Since loading of the tanks, vehicles and stores of the second wave onto the returned barges and transport ships would take at least a week, the second wave could not be expected to land much less than ten days after the first wave, and more likely longer still.[64]

Two types of inland river barge were generally available in Europe for use in Sea Lion: the peniche, which was 38.5 meters long and carried 360 tons of cargo, and the Kampine, which was 50 meters long and carried 620 tons of cargo. Of the barges collected for the invasion, 1,336 were classified as peniches and 982 as Kampinen. For simplicity's sake, the Germans designated any barge up to the size of a standard peniche as Type A1 and anything larger as Type A2.[65]

Converting the assembled barges into landing craft involved cutting an opening in the bow for off-loading troops and vehicles, welding longitudinal I-beams and transverse braces to the hull to improve seaworthiness, adding a wooden internal ramp and pouring a concrete floor in the hold to allow for tank transport. As modified, the Type A1 barge could accommodate three medium tanks while the Type A2 could carry four.[66] Tanks, armoured vehicles and artillery were envisaged as crossing the Channel in one of around 170 transport ships, which would be anchored off the landing beaches while the barges disembarked the first echelon of assault troops; those in powered barges disembarking soonest. The empty barges would then have been retrieved by tugs on the following rising tide, so as to have the second echelon (including tanks and other heavy equipment) loaded onto them using ship's derricks. Barges would consequently have shuttled between ships and beaches over at least two days before being assembled together for the escorted night-time return voyage across the Channel.

This barge was a Type A altered to carry and rapidly off-load the submersible tanks (Tauchpanzer) developed for use in Sea Lion. They had the advantage of being able to unload their tanks directly into water up to 15 metres (49 ft) in depth, several hundred yards from shore, whereas the unmodified Type A had to be firmly grounded on the beach, making it more vulnerable to enemy fire. The Type B required a longer external ramp (11 meters) with a float attached to the front of it. Once the barge anchored, the crew would extend the internally stowed ramp using block and tackle sets until it was resting on the water's surface. As the first tank rolled forward onto the ramp, its weight would tilt the forward end of the ramp into the water and push it down onto the seabed. Once the tank rolled off, the ramp would bob back up to a horizontal position, ready for the next one to exit. If a barge was securely grounded along its full length, the longer ramp could also be used to discharge submersible tanks directly onto the beach, and beachmasters were given the option of landing tanks by this method, if the risk of loss in submersible running appeared to be too high. The Navy High Command increased its initial order for 60 of these vessels to 70 in order to compensate for expected losses. A further five were ordered on 30 September as a reserve.[67]

The Type C barge was specifically converted to carry the Panzer II amphibious tank (Schwimmpanzer). Because of the extra width of the floats attached to this tank, cutting a broad exit ramp into the bow of the barge was not considered advisable as it would have compromised the vessel's seaworthiness to an unacceptable degree. Instead, a large hatch was cut into the stern, thereby allowing the tanks to drive directly into deep water before turning under their own motive power and heading towards shore. The Type C barge could accommodate up to four Schwimmpanzern in its hold. Approximately 14 of these craft were available by the end of September.[68]

During the planning stages of Sea Lion, it was deemed desirable to provide the advanced infantry detachments (making the initial landings) with greater protection from small-arms and light artillery fire by lining the sides of a powered Type A barge with concrete. Wooden slides were also installed along the barge's hull to accommodate ten assault boats (Sturmboote), each capable of carrying six infantrymen and powered by a 30 hp outboard motor. The extra weight of this additional armour and equipment reduced the barge's load capacity to 40 tons. By mid-August, 18 of these craft, designated Type AS, had been converted, and another five were ordered on 30 September.[66]

The Luftwaffe had formed its own special command (Sonderkommando) under Major Fritz Siebel to investigate the production of landing craft for Sea Lion. Major Siebel proposed giving the unpowered Type A barges their own motive power by installing a pair of surplus 600-horsepower (450-kilowatt) BMW aircraft engines, driving propellers. The Kriegsmarine was highly sceptical of this venture, but the Heer (Army) high command enthusiastically embraced the concept and Siebel proceeded with the conversions.[69]

The aircraft engines were mounted on a platform supported by iron scaffolding at the aft end of the vessel. Cooling water was stored in tanks mounted above-deck. As completed, the Type AF had a speed of six knots (11 km/h), and a range of 60 nautical miles (110 kilometres) unless auxiliary fuel tanks were fitted. Disadvantages of this set-up included an inability to back the vessel astern, limited manoeuvrability and the deafening noise of the engines which would have made voice commands problematic.[69]

By 1 October 128 Type A barges had been converted to airscrew propulsion and, by the end of the month, this figure had risen to over 200.[70]

The Kriegsmarine later used some of the motorised Sea Lion barges for landings on the Russian-held Baltic islands in 1941 and, though most of them were eventually returned to the inland rivers they originally plied, a reserve was kept for military transport duties and for filling out amphibious flotillas.[71]

As a consequence of employing all of their available cruisers in the North Sea deception operation, there would have been only light forces available to protect the vulnerable transport fleets. The plan revised on 14 September 1940 by Admiral Günther Lütjens called for three groups of five U-boats, all seven destroyers, and seventeen torpedo boats to operate to the west of the mine barrier in the Channel, while two groups of three U-boats and all the available E-boats to operate north of it.[72] Lütjens suggested the inclusion of the old battleships SMS Schlesien and SMS Schleswig-Holstein which were used for training. They were considered too vulnerable to send into action without improvement, especially considering the fate of their sister ship, SMS Pommern, which had blown up at the Battle of Jutland. The Blohm und Voss shipyard considered that it would take six weeks for a minimal upgrade of armour and armament and the idea was dropped, as was a suggestion that they be used as troopships.[73] Four coasters were converted to auxiliary gunboats by the addition of a single 15 cm naval gun and another was fitted with two 10.5 cm guns, while a further twenty-seven smaller vessels were converted into light gunboats by attaching a single ex-French 75 mm field gun to an improvised platform; these were expected to provide naval gunfire support as well as fleet defence against modern British cruisers and destroyers.[74]

Providing armour support for the initial wave of assault troops was a critical concern for Sea Lion planners, and much effort was devoted to finding practical ways of rapidly getting tanks onto the invasion beaches in support of the first echelon. Though the Type A barges could disembark several medium tanks onto an open beach, this could be accomplished only once the tide had fallen further and the barges were firmly grounded along their full length; otherwise a leading tank might topple off an unsteady ramp and block those behind from deployment. The time needed for assembling the external ramps also meant that both the tanks and the ramp assembly crews would be exposed to close-quarter enemy fire for a considerable time. A safer and faster method was needed and the Germans eventually settled on providing some tanks with floats and making others fully submersible. It was nevertheless recognised that a high proportion of these specialised tanks might be expected not to make it off the beach.

The Schwimmpanzer II Panzer II, at 8.9 tons, was light enough to float with the attachment of long rectangular buoyancy boxes on each side of the tank's hull. The boxes were machined from aluminium stock and filled with Kapok sacks for added buoyancy. Motive power came from the tank's own tracks which were connected by rods to a propeller shaft running through each float. The Schwimmpanzer II could make 5.7 km/h in the water. An inflatable rubber hose around the turret ring created a waterproof seal between the hull and turret. The tank's 2 cm gun and coaxial machinegun were kept operational and could be fired while the tank was still making its way ashore. Because of the great width of the pontoons, Schwimmpanzer IIs were to be deployed from specially-modified Type C landing barges, from which they could be launched directly into open water from a large hatch cut into the stern. The Germans converted 52 of these tanks to amphibious use prior to Sea Lion's cancellation.[75]

A Panzer III Tauchpanzer under test (1940)

The Tauchpanzer or deep-wading tank (also referred to as the U-Panzer or Unterwasser Panzer) was a standard Panzer III or Panzer IV medium tank with its hull made completely waterproof by sealing all sighting ports, hatches and air intakes with tape or caulk. The gap between the turret and hull was sealed with an inflatable hose while the main gun mantlet, commander's cupola and radio operator's machine gun were given special rubber coverings. Once the tank reached the shore, all covers and seals could be blown off via explosive cables, enabling normal combat operation.[76]

Fresh air for both the crew and engine was drawn into the tank via an 18 m long rubber hose to which a float was attached to keep one end above the water's surface. A radio antenna was also attached to the float to provide communication between the tank crew and the transport barge. The tank's engine was converted to be cooled with seawater, and the exhaust pipes were fitted with overpressure valves. Any water seeping into the tank's hull could be expelled by an internal bilge pump. Navigation underwater was accomplished using a directional gyrocompass or by following instructions radioed from the transport barge.[76]

Experiments conducted at the end of June and early July at Schilling, near Wilhelmshaven, showed that the submersible tanks functioned best when they were kept moving along the seabed as, if halted for any reason, they tended to sink into the seabed and remain stuck there. Obstacles such as underwater trenches or large rocks tended to stop the tanks in their tracks, and it was decided for this reason that they should be landed at high tide so that any mired tanks could be retrieved at low tide. Submersible tanks could operate in water up to a depth of 15 metres (49 ft).[77]

The Kriegsmarine initially expected to use 50 specially-converted motor coasters to transport the submersible tanks, but testing with the coaster Germania showed this to be impractical. This was due to the ballast needed to offset the weight of the tanks, and the requirement that the coasters be grounded to prevent them from capsizing as the tanks were transferred by crane onto the vessel's wooden side ramps. These difficulties led to development of the Type B barge.[77]

By the end of August the Germans had converted 160 Panzer IIIs, 42 Panzer IVs, and 52 Panzer IIs to amphibious use. This gave them a paper strength of 254 machines, about an equivalent number to those that would otherwise have been allocated to an armoured division. The tanks were divided into four battalions or detachments labelled Panzer-Abteilung A, B, C and D. They were to carry sufficient fuel and ammunition for a combat radius of 200 km.[78]

As part of a Kriegsmarine competition, prototypes for a prefabricated "heavy landing bridge" or jetty (similar in function to later Allied Mulberry Harbours) were designed and built by Krupp Stahlbau and Dortmunder Union and successfully overwintered in the North Sea in 1941–42.[79] Krupp's design won out, as it only required one day to install, as opposed to twenty-eight days for the Dortmunder Union bridge. The Krupp bridge consisted of a series of 32m-long connecting platforms, each supported on the seabed by four steel columns. The platforms could be raised or lowered by heavy-duty winches in order to accommodate the tide. The German Navy initially ordered eight complete Krupp units composed of six platforms each. This was reduced to six units by the autumn of 1941, and eventually cancelled altogether when it became apparent that Sea Lion would never take place.[80]

In mid-1942, both the Krupp and Dortmunder prototypes were shipped to the Channel Islands and installed together off Alderney, where they were used for unloading materials needed to fortify the island. Referred to as the "German jetty" by local inhabitants, they remained standing for the next thirty-six years until demolition crews finally removed them in 1978–79, a testament to their durability.[80]

The German Army developed a portable landing bridge of its own nicknamed Seeschlange (Sea Snake). This "floating roadway" was formed from a series of joined modules that could be towed into place to act as a temporary jetty. Moored ships could then either unload their cargo directly onto the roadbed or lower it down onto waiting vehicles via their heavy-duty booms. The Seeschlange was successfully tested by the Army Training Unit at Le Havre in France in the autumn of 1941 and later chosen for use in Operation Herkules, the proposed Italo-German invasion of Malta. It was easily transportable by rail.[80]

A specialised vehicle intended for Sea Lion was the Landwasserschlepper (LWS), an amphibious tractor under development since 1935. It was originally intended for use by Army engineers to assist with river crossings. Three of them were assigned to Tank Detachment 100 as part of the invasion; it was intended to use them for pulling ashore unpowered assault barges and towing vehicles across the beaches. They would also have been used to carry supplies directly ashore during the six hours of falling tide when the barges were grounded. This involved towing a Kässbohrer amphibious trailer capable of transporting 10–20 tons of freight behind the LWS.[81] The LWS was demonstrated to General Halder on 2 August 1940 by the Reinhardt Trials Staff on the island of Sylt and, though he was critical of its high silhouette on land, he recognised the overall usefulness of the design. It was proposed to build enough tractors that one or two could be assigned to each invasion barge, but the late date and difficulties in mass-producing the vehicle prevented this.[81]

Operation Sea Lion would have been the first ever amphibious invasion by a mechanised army, and the largest amphibious invasion since Gallipoli. The Germans had to invent and improvise a lot of equipment. They also proposed to use some new weapons and use upgrades of their existing equipment for the first time. These included:

1. New antitank guns and ammunition. The standard German antitank gun, the 37 mm Pak 36, was capable of penetrating the armour of all 1940 British tanks except the Matilda and Valentine. Armour-piercing ballistic capped (tungsten-cored) ammunition (Pzgr. 40) for 37 mm Pak 36 had become available in time for the invasion.[82] The 37 mm Pzgr.40 would still have had trouble penetrating the Matilda II's armour[83] so the first echelon units replaced theirs with French or Czechoslovak 47 mm guns (which weren't much better).[84] The Pak 36 began to be replaced by the 50 mm Pak 38 in mid-1940. The Pak 38, which could penetrate a Matilda's armour, would probably have seen action first with Sea Lion as it would have been issued initially to the Waffen-SS and the Heer's elite units, and all those units were in the Sea Lion force. These included the SS Leibstandarte Adolf Hitler regiment, the Großdeutschland regiment, 2 mountain, 2 Jäger, 2 Fallschirmjäger, 4 panzer, and 2 motorised divisions. In addition, the 7th Infantry division was considered one of the best in the Heer, and the 35th almost as good.
2. Captured French armoured tractors.[85] The use of these tractors by the first wave units was intended to reduce their dependence upon horses and probably would have reduced the problems of getting supplies off the beaches. In addition to their proposed use on the beaches, the Germans later used them as tractors for antitank guns and munitions carriers, as self-propelled guns, and as armoured personnel carriers. There were two main types. The Renault UE Chenillette (German name: Infanterie Schlepper UE 630 (f)) was a light tracked armoured carrier and prime mover produced by France between 1932 and 1940. Five to six thousand were built, and about 3,000 were captured and overhauled by the Germans.[86] They had a storage compartment that could carry 350 kg, pull a trailer weighing 775 kg for a total of about 1000 kg, and could climb a 50% slope. The armour was 5–9 mm, enough to stop shell fragments and bullets. There was also the Lorraine 37L, which was larger, of which 360 fell into German hands. In that vehicle a load of 810 kilograms could be carried, plus a 690 kg trailer pulled for a total of 1.5 tonnes. The use of such captured equipment meant that the first wave divisions were largely motorised,[84] with the first wave using 9.3% (4,200) of the 45,000 horses normally required.[87]
3. 48× Stug III Ausf B Assault Guns – 7.5 cm StuK 37 L/24, 50 mm armour and improved suspension. Some were to be landed with the first wave.[88]
4. Panzer III F/G upgraded with more armour on the mantlet and progressively from 3.7 cm KwK 36 L/46.5 to 5 cm KwK 38 L/42.
5. 72 Nebelwerfer, to be landed with the second and third waves.[89]
6. 36× Flammpanzer II flamethrower tanks, 20 to land with the first wave.[89]
7. 4 or more 75 mm Leichtgeschütz 40 recoilless guns, for use by paratroopers. The LG 40 could be split into four parts with each part being dropped on a single parachute.[90]

Four years later, the Allied D-Day landings showed just how much materiel had to be landed continuously to maintain an amphibious invasion. The problem for the Germans was worse, as the German Army was mostly horse-drawn. One of its prime headaches would have been transporting thousands of horses across the Channel.[133] British intelligence calculated that the first wave of 10 divisions (including the airborne division) would require a daily average of 3,300 tons of supplies.[134] In fact, in Russia in 1941, when engaged in heavy fighting (at the end of a very long supply line), a single German infantry division required up to 1,100 tons of supplies a day,[135] though a more usual figure would be 212–425 tons per day.[136] The smaller figure is more likely due to the very short distances the supplies would have to travel. Rations for two weeks were to be provided to the German troops of the first wave because the armies had been instructed to live off the land as far as possible in order to minimise supply across the Channel during the initial phase of the battle.[137] British intelligence further calculated that Folkestone, the largest harbour falling within the planned German landing zones, could handle 150 tons per day in the first week of the invasion (assuming all dockside equipment was successfully demolished and regular RAF bombing raids reduced capacity by 50%). Within seven days, maximum capacity was expected to rise to 600 tons per day, once German shore parties had made repairs to the quays and cleared the harbour of any blockships and other obstacles. This meant that, at best, the nine German infantry and one airborne division landed in the first wave would receive less than 20% of the 3,300 tons of supplies they required each day through a port, and would have to rely heavily on whatever could be brought in directly over the beaches or air-lifted into captured airstrips.[138]

The successful capture of Dover and its harbour facilities might have been expected to add another 800 tons per day, raising to 40% the amount of supplies brought in through ports. However, this rested on the rather unrealistic assumption of little or no interference from the Royal Navy and RAF with the German supply convoys which would have been made up of underpowered (or unpowered, i.e. towed) inland waterways vessels as they shuttled slowly between the Continent to the invasion beaches and any captured harbours.[138]

From 19 to 26 September 1940, sea and wind conditions on and over the Channel where the invasion was to take place were good overall, and a crossing, even using converted river barges, was feasible provided the sea state remained at less than 4, which for the most part it did. Winds for the remainder of the month were rated as "moderate" and would not have prevented the German invasion fleet from successfully depositing the first wave troops ashore during the ten days needed to accomplish this.[139] From the night of 27 September, strong northerly winds prevailed, making passage more hazardous, but calm conditions returned on 11–12 October and again on 16–20 October. After that, light easterly winds prevailed which would have assisted any invasion craft travelling from the Continent towards the invasion beaches. But by the end of October, according to British Air Ministry records, very strong south-west winds (force 8) would have prohibited any non-seagoing craft from risking a Channel crossing.[140]

At least 20 spies were sent to Britain by boat or parachute to gather information on the British coastal defences under the codename "Operation Lena"; many of the agents spoke limited English. All agents were quickly captured and many were convinced to defect by MI5's Double-Cross System, providing disinformation to their German superiors. It has been suggested that the "amateurish" espionage efforts were a result of deliberate sabotage by the head of the army intelligence bureau in Hamburg, Herbert Wichmann, in an effort to prevent a disastrous and costly amphibious invasion; Wichmann was critical of the Nazi regime and had close ties to Wilhelm Canaris, the head of the Abwehr, the German military intelligence agency.[141]

While some errors might not have caused problems, others, such as the inclusion of bridges that no longer existed[142] and misunderstanding the usefulness of minor British roads,[142] would have been detrimental to German operations, and would have added to the confusion caused by the layout of Britain's cities (with their maze of narrow roads and alleys) and the removal of road signs.[143]

A 1974 wargame was conducted at Royal Military Academy Sandhurst.[144] The controllers of the game assumed that the Luftwaffe had not diverted its daytime operations into bombing London on 7 September 1940, but had continued its assault against RAF airbases in the South East. Consequently, the German High Command, relying on grossly overstated claims of RAF fighters shot down, were under the erroneous impression that by 19 September RAF front-line fighter strength had fallen to 140 (as against a true figure of over 700); and hence that effective German air superiority might shortly be achieved.[145] In the Game, the Germans were able to land almost all their first echelon forces on 22 September 1940, and established a beachhead in south-east England, capturing Folkestone, and Newhaven, even though the British had demolished the facilities of both ports. The British army forces, delayed in moving units from East Anglia to the South East by bomb damage to the rail network south of London, were nevertheless able to hold onto positions in and around Newhaven and Dover, sufficient to deny their use by German forces. Both the RAF and the Luftwaffe lost nearly a quarter of their available forces on the first day, after which it finally became apparent to the German command that British airpower was not, after all, on the point of collapse. On the night of 23/24 September a Royal Navy force of cruisers and destroyers was able to reach the Channel from Rosyth, in time to intercept and destroy most of the barges carrying the second and third echelon of German amphibious landings with the crucial tanks and heavy artillery (for the game, these follow-up echelons had been held back from crossing the Channel on S minus one with the first echelon, instead sailing across on the night of S plus one). Without the second and third echelons, the forces ashore were cut off from reserves of artillery, vehicles, fuel and ammunition supplies; and blocked from further reinforcements. Isolated and facing fresh regular troops with armour and artillery, the invasion force was forced to surrender after six days.[146]

One of the primary German foreign policy aims throughout the 1930s had been to establish a military alliance with the United Kingdom, and despite anti-British policies having been adopted as this proved impossible, hope remained that the UK would in time yet become a reliable German ally.[147] Hitler professed an admiration for the British Empire and preferred to see it preserved as a world power, mostly because its break-up would benefit other countries far more than it would Germany, particularly the United States and Japan.[147][148] Britain's situation was likened to the historical situation of the Austrian Empire after its defeat by the Kingdom of Prussia in 1866, after which Austria was formally excluded from German affairs but would prove to become a loyal ally of the German Empire in the pre-World War I power alignments in Europe. It was hoped that a defeated Britain would fulfill a similar role, being excluded from continental affairs, but maintaining its Empire and becoming an allied seafaring partner of the Germans.[147][149]

The continued military actions against the UK after the fall of France had the strategic goal of making Britain 'see the light' and conduct an armistice with the Axis powers, with 1 July 1940 being named the "probable date" for the cessation of hostilities.[150] On 21 May 1940, Chief of Army Staff Franz Halder, after a consultation with Hitler on the war aims regarding Britain, wrote in his diary: "We are seeking contact with Britain on the basis of partitioning the world".[151] Even as the war went on Hitler hoped in August 1941 for the eventual day when "England and Germany [march] together against America", and in January 1942 he still daydreamed that it was "not impossible" for Britain to quit the war and join the Axis side.[152] Nazi ideologist Alfred Rosenberg hoped that after the victorious conclusion of the war against the USSR, Englishmen would be among the Germanic nationalities who would join the Germanic settlers in colonising the conquered eastern territories.[153]

Other evidence suggests that in the case of a successful invasion of Great Britain the occupiers treatment of the British population may not have been as sympathetic. According to captured German documents, the commander-in-chief of the German Army, Walther von Brauchitsch, directed that "The able-bodied male population between the ages of 17 and 45 will, unless the local situation calls for an exceptional ruling, be interned and dispatched to the Continent". The remaining population would have been terrorised, including civilian hostages being taken and the death penalty immediately imposed for even the most trivial acts of resistance, with the UK being plundered for anything of financial, military, industrial or cultural value.[154] After the war Otto Bräutigam of the Reich Ministry for the Occupied Eastern Territories wrote in his book that he had encountered a personal report by General Eduard Wagner regarding a discussion with Heinrich Himmler from February 1943, in which Himmler had expressed the intention for Einsatzgruppen to kill about 80% of the populations of France and England after the German victory.[155] At another point, Hitler had on one occasion described the English lower classes "racially inferior".[156]

According to the most detailed plans created for the immediate post-invasion administration, Great Britain and Ireland were to be divided into six military-economic commands, with headquarters in London, Birmingham, Newcastle, Liverpool, Glasgow and Dublin.[157] Hitler decreed that Blenheim Palace, the ancestral home of Winston Churchill, was to serve as the overall headquarters of the German occupation military government.[158] The OKW, RSHA, and Foreign Ministry compiled lists of those they thought could be trusted to form a new German-friendly government along the lines of the one in occupied Norway. The list was headed by British fascist leader Oswald Mosley. The RSHA also felt that Harold Nicolson might prove useful in this role.[159] It appears, based on the German police plans, that the occupation was to be only temporary, as detailed provisions for the post-occupation period are mentioned.[160]

Some sources indicated that the Germans only intended to occupy Southern England, and that draft documents existed on the regulation of the passage of British civilians back and forth between the occupied and unoccupied territories.[161] Others state that Nazi planners envisaged the institution of a nationalities policy in Western Europe to secure German hegemony there, which entailed the granting of independence to various regions. This involved detaching Scotland from the United Kingdom, the creation of a United Ireland, and an autonomous status for Western England.[162]

After the war rumours also emerged about the selection of either Joachim von Ribbentrop or Ernst Wilhelm Bohle, for the "viceregal" office of Reichskommissar für Großbritannien ("Imperial Commissioner for Great Britain").[163] However, no establishment by this name was ever approved by either Hitler or the Nazi government during the war, and was also denied by Bohle when he was interrogated by the victorious Allies (von Ribbentrop not having been questioned on the matter). After the Second Armistice at Compiègne with France, when he expected an imminent British capitulation, Hitler did however assure Bohle that he would be the next German ambassador to the Court of St. James's "if the British behave[d] sensibly".[163]

Edward, Duke of Windsor reviewing SS guards with Robert Ley, 1937

The German Government used 90% of James Vincent Murphy's rough draft translation of Mein Kampf to form the body of an edition to be distributed in the UK once Operation Sea Lion was completed. This 'Operation Sea Lion Edition' was finalised and printed in the summer of 1940. Once the invasion was called off by Adolf Hitler most copies were distributed to English speaking POW camps. Original copies are very rare and highly sought after by serious book collectors interested in military history.

A Channel 5 documentary broadcast on 16 July 2009 repeated the claim that the Germans intended to restore Edward VIII to the throne in the event of a German occupation.[164][165] Many senior German officials believed the Duke of Windsor to be highly sympathetic to the Nazi government, a feeling that was reinforced by his and Wallis Simpson's 1937 visit to Germany. However, the Foreign Office maintains that despite German approaches; "The Duke never wavered in his loyalty to Great Britain during the war".[166]

Franz Six in 1940. He was later convicted of war crimes in the Nuremberg trials.

Had Operation Sea Lion succeeded, Franz Six was intended to become the Sicherheitsdienst (SD) Commander in the country, with his headquarters to be located in London, and with regional task forces in Birmingham, Liverpool, Manchester, and Edinburgh.[157] His immediate mission would have been to hunt down and arrest the 2,820 people on the Sonderfahndungsliste G.B. ("Special Search List Great Britain"). This document, which post-war became known as "The Black Book", was a secret list compiled by Walter Schellenberg containing the names of prominent British residents to be arrested immediately after a successful invasion.[167] Six would also have been responsible for handling the large population of British Jews, over 300,000 at that time.[167]

Six had also been entrusted with the task of securing "aero-technological research result and important equipment" as well as "Germanic works of art". There is also a suggestion that he toyed with the idea of moving Nelson's Column to Berlin.[168] The RSHA planned to take over the Ministry of Information, to close the major news agencies and to take control of all of the newspapers. Anti-German newspapers were to be closed down.[169]